I've never heard that. The small grain that might affect blood donation eligibility is if they have traveled outside the U.S. a lot or to certain countries. You may be deferred if your travel puts you at risk to carry malaria or a variant of the mad cow virus.

I could possibly see there being an effect from exposure to gamma rays. Most of those are absorbed in the troposphere (lowest level of atmosphere), and planes fly higher than the bulk (by mass, not depth) of the troposphere. That exposure for so many hours every week could potentially damage blood and other organs, probably temporarily, but potentially enough to require deferral. But I have never previously heard this rumor either.